1. Field of the Invention
This invention relates to an electrophotographic photosensitive member, a process cartridge and an electrophotographic apparatus which have the electrophotographic photosensitive member. More particularly, it relates to an electrophotographic photosensitive member having a surface layer containing a resin with a specific structure, and a process cartridge and an electrophotographic apparatus which have such an electrophotographic photosensitive member.
2. Related Background Art
Inorganic materials such as selenium, cadmium sulfide and zinc oxide are conventionally known as photoconductive materials used in electrophotographic photosensitive members. In contrast, organic materials including polyvinyl carbazole, phthalocyanine and azo pigments have attracted attention due to the advantages that they promise, such as high productivity and no environmental pollution. They have been put into wide use although they tend to be inferior to the inorganic materials in photoconductive performance or running performance.
Meanwhile, electrophotographic photosensitive members are required to be durable against various external physical, chemical and electrical forces, since they are repeatedly affected by charging, exposure, development, transfer, cleaning and charge elimination in electrophotographic processes in copying machines or laser beam printers. In particular, the surface layer of the photosensitive member, i.e., the layer most distant from the support is required to have durability to surface wear and scratching which are caused by, e.g., rubbing, and is also required to be durable against surface deterioration caused by charging.
In image forming apparatus of an electrophotographic system, corona charging assemblies have been used as means for electrostatically charging the electrophotographic photosensitive member. In this system, corona products such as ozone and nitrogen oxides are formed when corona occurs, and this accelerates the deterioration of the photosensitive member's surface.
In recent years, because of low ozone and low power consumption, apparatus are used in which the photosensitive member is charged by applying a voltage to a charging member coming in contact with the photosensitive member, i.e., a contact charging assembly. Specifically, the photosensitive member is charged by the discharge caused at a minute gap between the charging member and the photosensitive member by applying a voltage of about 1 to 2 kV between the charging member and the photosensitive member.
However, in the system where only a DC voltage is applied to the charging member, the resistivity of the charging member may vary depending on variations of the temperature and humidity that surround the apparatus. Also, the electrostatic capacity of the photosensitive member may vary as a result of a change in layer thickness caused by scrape because of repeated use. Hence, it is difficult to keep the surface potential of the photosensitive member at the desired value.
Accordingly, in order to achieve the uniformity of charging, a method is used in which an AC voltage having a peak-to-peak voltage at least twice the discharge threshold voltage is superimposed on the DC voltage corresponding to the desired charging voltage.
However, even the contact charging produces ozone in a very small quantity. Since the discharge takes place in the vicinity of the photosensitive member, the damage to the photosensitive member is much greater than that caused by the corona discharge. This damage is even greater when the system of superimposing AC voltage is used. Thus, the surface layer deterioration due to charging has more and more influence.
As stated above, the properties required for the surface layer are specifically exemplified by chemical resistance to ozone and nitrogen oxides occurring at the time of charging, electrical resistance to discharge, and mechanical strength against rubbing in, for example, cleaning. The scrape occurring in the contact charging system is so conspicuous that the surface deterioration caused by charging may have a substantial influence, and an improvement in these properties is sought.